Add basic error handling framework and 404 handling

[RichDom2185]

• Added basic error handling framework
• Do not crash server (panic) on errors

Part of #49.

Only updated one controller as a minimal example. Remaining controllers will be updated in a future PR

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Overview

There are 2 kinds of errors:

• Client errors: Not Found, Bad Request, etc..
• Everything else: Internal Server Error

We use a custom error interface ClientError for our errors that have a custom status code. This allows us to create custom error messages, such as those in HandleDBError.

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Screenshots

Previously, reading an invalid user gives the wrong status code and output:

Now, reading an invalid user will give the correct result:

[github-actions[bot]]

coverage: 60.976% (-3.5%) from 64.516% when pulling 1291080eec6338185752fa4c8eea16c00af52648 on add-basic-404-handling into 5bd4e88efd0c4a8b52fc897cda14cd7d18470b69 on main.